MXS-1777: Add an EMAverage to the server struct, and a new slave selection criteria.

This is to support calculating the average from a session, and the slave selection criteria to be able to route based on averages. This commit, like the next one, have TODOs which you should feel free to comment on. Undecided things.

maxutils/maxbase/src/average.cc

@@ -62,7 +62,9 @@ void CumulativeAverage::reset()
 }
 
 EMAverage::EMAverage(double min_alpha, double max_alpha, int sample_max) :
-    m_min_alpha{min_alpha}, m_max_alpha{max_alpha}, m_sample_max{sample_max}
+    m_min_alpha{min_alpha},
+    m_max_alpha{max_alpha},
+    m_sample_max{sample_max}
 {
 }